Bacillus thuringiensis (“Bt”) strains are rich sources of insect inhibitory toxin proteins. These proteins can be used to control agriculturally-relevant pests of crop plants by spraying formulations containing these proteins onto plants or by expressing these proteins in plants and seeds. Notably, only a few toxic proteins are currently used commercially for controlling insects of the order Coleoptera, such as corn rootworms. Such currently deployed toxic proteins include Cry3Bb1, a modified Cry3A, eCry3.1Ab, and a binary toxin Cry34Ab1/Cry35Ab1 (requiring two different proteins for toxic activity). These proteins are effective for controlling Diabrotica species that infest corn roots, whether deployed singly, or in various combinations to decrease the likelihood of the development of resistance. Even though these proteins have been successfully deployed as insect control agents in transgenic crop plants, resistance to their effects can develop.
Resistance to a deployed toxin, whether chemistry or protein, is more likely to develop in a number of situations which enhance resistance development. Generally, the development of resistance is directly dependent on the length of time that a toxin is deployed into the environment. Resistance development is also more likely to increase in situations in which the dose of the toxin is insufficient to ensure mortality to the pest consuming a single bite of tissue containing the toxin. Accordingly, it is crucial to deliver a lethal dose of toxin with each bite, otherwise development of resistance to a particular toxin is more likely to occur. Repetitive use of the same toxin within a common geographic region on or in multiple species of plants which are susceptible to the same or similar pests within a common geographic region is more likely to cause rapid development of resistance to the toxin, particularly in climates in which there are multiple generations of a particular target pest within a single growing season. Geographic regions that are nearer to the equator tend to have longer and more consistent photoperiods and climates conducive for plant growth and cultivation, and the pests in these regions tend to have multiple generations in shorter periods of time than in other locations which are exposed to more dramatic climate changes that reduce the number of reproductive cycles that are encountered in similar periods of time. For all the forgoing reasons, dependence on a limited number of toxic proteins or toxic chemistries can result in the development of resistance to these pest control agents.
Other proteins disclosed in the art that are asserted to exhibit toxic effects to corn rootworms include patatin, TIC100/101 binary toxin, ET33/34 binary toxin, TIC863, ET80/76 binary toxin, ET70, Cry3Bb (U.S. Pat. No. 6,501,009), Cry1C variants, Cry3A variants, Cry3, Cry3B, Cry34/35, 5307, Axmi184, Axmi205, AxmiR1, TIC901, TIC1201, TIC407, TIC417, TIC431, TIC807, TIC853, TIC3131, DIG-10, eHIPs (U.S. Patent Application Publication No. 2010/0017914), and ω-Hexatoxin-Hv1a (U.S. Patent Application Publication US2014-0366227 A1). These proteins may be provided alone or in combinations with other toxic agents in subsequent commercial embodiments to insure durability of the rootworm resistant product and to decrease the likelihood of resistance development. However, none of these additional proteins have been observed to provide the low dose toxic effect against corn rootworms that the currently deployed commercial toxin proteins exhibit, and so have not been recognized to be commercially useful.
Accordingly, there is a need in the art for the discovery, development, and commercial deployment of new toxic agents, particularly environmentally friendly, highly selective and specific toxin proteins or chemistries that are active when used either alone or in various combinations with one or more supplementary toxic agents against a broad spectrum of insect pest species, particularly those pest species—such as Coleopteran species—that have been observed to develop resistance to currently deployed toxins.